The present invention relates generally to an improved optical system for shaping illuminating beams from continuous duty searchlights or the like so as to minimize the effect of backscattered illumination and, thereby enhance the brightness contrast between illuminated target objects and background so as to improve visibility; and, more particularly, to methods and apparatus comprising an optical system of the type employing a continuous duty searchlights wherein the distribution of the light beam emanating from the aperture of a lamp--i.e., the entrance pupil of the optical system--comprises a collimated beam of high radiant intensity and, wherein provision is made for splitting such collimated beam along a line bisecting the distributional pattern of light, with respective halves of the distribution being first directed in opposite directions and then being redirected in substantially parallel directions and, wherein the light from each half of the split distribution is imaged into respective ones of a pair of collimating lenses with each lens receiving light only from the upper half of its focal plane so that the parallel beams leaving the exit pupils of the optical system contain light only in the space below a generally horizontal plane passing through and containing the optical axes of the system.
Various types of situations exist where visibility under adverse meterologic conditions is critical. A typical, but non-limiting, example of such a situation is that involving movement of marine vessels over heavily trafficked waterways such as a harbor, a harbor entrance, and the like. Often such marine activities must be carried out at night and under weather conditions where fog or the like is prevalent. Such vessels commonly employ continuous duty searchlights to illuminate the path along which the vessel is to pass, with the illuminated path being continuously monitored by, for example, low light level television cameras or, in some instances, simply by individuals functioning as "lock-outs". In either case, the ability to discriminate between target objects and background in the path of vessel movement can be, and is, seriously degraded by backscattered illumination which serves to decrease and, in many cases, to virtually eliminate the brightness contrast between illuminated target objects and background, thus making it virtually impossible to detect objects in the path of movement in time to permit suitable evasive action.
The problem typically encountered is one with which most operators of motor vehicles are familiar. Thus, it can be analogized to driving an automobile at night under foggy conditions where the operator's eye level is located only a few feet above the level of the headlights. When the headlights are switched from "low beam" to "high beam", visibility is seriously impaired due to backscattered illumination from the miniscule droplets of moisture in the air. If, on the other hand, it were possible to locate the driver directly above the portion of the road being illuminated so that he could view the illuminated area without having to look along the beam from the headlights, backscattered light would pose no significant problem. It is for this reason that operators of tractor-trailers and the like enjoy better visibility under foggy conditions than do drivers of automobiles--viz., the truck drivers are generally located in the relatively high cab of their vehicles at an eye level which is on the order of up to six to eight feet above the level of the truck headlights. However, even under these conditions backscattered illumination still poses a problem; this despite the fact that the area illuminated by vehicular headlights extends for only a few hundred feet ahead of the vehicle. It will, of course, be understood by those skilled in the art that as the range of the illuminated area is increased, the included angle between the line of sight from the observer and the axis of the illuminating beam is decreased, thereby inherently requiring the observer to look along the illuminated path even though he may be several feet above the path.
Considering marine vessels where maneuverability is much more restricted than with motor vehicles it is necessary to illuminate an area extending many hundreds of feet--indeed, in many instances, an area extending up to, and even in excess of, a mile--in front of the vessel and, consequently, even when it is possible to locate a monitoring instrument such as a low light level television camera (or a human eye) ten, fifteen, or twenty feet, or more, above the light source, the monitoring instrument is still required to look along the path of illumination, particularly as the range increases. This produces serious visibility problems for the reasons desscribed above when using conventional continuous duty searchlights.
It should be understood that while the invention finds particularly advantageous use in connection with minimizing illumination backscatter on marine vessels such, for example, as relatively high-speed hydrofoil vessels or the like, and is therefore herein described in such an environment, it is not necessarily restricted to marine applications; but, rather, the invention may find use in other areas where backscattered illumination presents a problem and the particular economics involved permit the use of the invention to improve visibility. That is, the present invention would, if economically feasible, improve visibility for operators of motor vehicles; but, for all practical purposes, the cost of installing such a system in most motor vehicles would be prohibitively high and, therefore, not justified. However, the cost involved might very well be justified in other situations such as movement of large, heavy vehicles, illuminating systems for assisting pilots to land aircraft under conditions of limited visibility, and similar situations.